The effect of photodynamic therapy using mono-L-aspartyl chlorin e6 (NPe6) on both direct cytotoxicity and vascular damage was examined. Sprague-Dawley rats bearing chondrosarcoma tumor were given i.v. injections of 5 or 10 mg/kg NPe6 and exposed to 135-J/cm2 664-nm laser light either 4 or 24h after NPe6 injection. The percentage of viable tumor cells was estimated either immediately after the completion of light treatment or 24 h after treatment using a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Measurements of arteriole constriction and venule leakage in normal cremaster tissues were made during and 1 h after the light treatment. Tumor response was evaluated for the 4 different NPe6 dose and time combinations. Both direct tumor cytotoxicity and vascular stasis were observed during light treatment. Vessel leakage did not occur. Blood flow stasis was a result of platelet aggregation and the mechanical obstruction of flow rather than vessel constriction. The magnitude of direct cytotoxicity and vascular response was dependent on both the amount of NPe6 delivered and the delay between injection and light treatment. Tumor cure was found in animals either when given high NPe6 doses or when treated early after NPe6 injection. Treatment regimens which maximized the effect of both vascular stasis and direct tumor cytotoxicity were found to produce the best tumor response. Dose combinations which produced vascular stasis with minimal early cytotoxicity did not result in cure. The combined mechanisms of damage after photodynamic therapy using NPe6 suggests that this photosensitizer may have specific advantages for clinical use and provides a benchmark for the development of new photosensitizers.